In many process applications, the composition of a gas must be carefully controlled to ensure that certain components in the gas are below a predetermined level or absent from the gas. Typically, the purified gaseous streams are either used as a component in a process operation, e.g. fluidic control systems, or to create an environment free of particular components, e.g. semiconductor chip manufacture. There are various systems available in the prior art to control the presence of undesirable components below a certain level.
Typically in prior art membrane gas dehydrators, compressed gas containing a permeate to be removed enters an inlet plenum at an elevated pressure. The gas then flows through the interior of a plurality of membrane capillary fibers. The size of the fiber pores allows the passage of the permeate but retards the passage of the gas. The permeate depleted gas enters the system for which it is designed.
The permeate is driven through the membrane walls by the difference in permeate vapor pressure between the inside (tube side) and outside (shell side) of the fibers. If no modifications are made to the system, the shell side of the fibers becomes permeate rich and the vapor pressure differential decreases.
In order to maintain a high vapor pressure differential the permeate is removed from the shell side of the fibers. It is conventional to provide a sweep stream or purge gas for this purpose. Typically a bypass stream of the permeate-depleted gas is recycled to purge the shell side.
Some membrane gas dehydrators are designed for `uncontrollable` discharge of the gases to the shell side. These dehydrators allow some gas to pass through the fibers along with the permeate. The required purge is created by simply venting the shell side to ambient. If the permeate being removed is corrosive, flammable, toxic, expensive or otherwise undesirable to exhaust to ambient a more sophisticated system which is less cost effective must be used.
My copending application Ser. No. 08/689,901 filed Aug. 15, 1996 overcomes this problem. A purge gas is used with this type of dehydrator, which purge gas is especially selected to neutralize the deleterious effects of the purge discharge while sweeping the shell side.
Another problem with this type of dehydrator is that with inlet conditions of pressure and moisture content (dewpoint) fixed, the moisture content of the outlet gas is determined by the balance between total flow rate and purge flow rate. The higher the ratio of total flow rate to purge flow rate, the higher the moisture content or dewpoint of the outlet air and contrariwise.
The purge flow rate is a fixed value at a given operating pressure. The outlet dewpoint will vary directly as the total flow. With a prior art dehydrator, operating at 80 psig and 60.degree. F. inlet dewpoint, the dehydrator delivers outlet dewpoints according to the following table:
TABLE ______________________________________ TOTAL FLOW PURGE FLOW NET FLOW OUTLET Standard Standard (Total - DEWPOINT cu ft/min cu ft/min Purge) .degree.F. ______________________________________ 22.0 2.7 19.3 40 13.1 2.7 10.4 20 10.0 2.7 7.3 0 8.4 2.7 5.7 -20 7.3 2.7 4.6 -40 ______________________________________
From this table it can be seen that if a process requires a net flow greater than 4.6 SCFM with an output dewpoints of -40.degree. F., two dehydrators would have to be operated in parallel. Parallel operation of multiple elements does provide increased outlet capacity, but at increased capital cost and with purge losses increased by multiples of the number of elements used. My present invention overcomes this problem.
The present invention is directed to an improvement in membrane dehydrators of the type based on `uncontrollable` discharge of gases from the shell side directly to ambient.
Broadly my invention comprises a system and method to modify the discharge characteristics on the shell side of an `uncontrollable` dehydrator to increase the capacity of the dehydrator. The invention comprises a dehydrator having a housing, a membrane disposed within the housing, the membrane defining a shell side and a tube side. An inlet introduces a gas into the tube side, the permeate and a portion of the gas flow through the walls of the tubes into the shell side. An outlet vents the shell side to ambient. A purge gas is introduced into the shell side and mixed with the permeate rich gas to increase the efficiency and performance of the dehydrator.
In the method of the invention, if the dehydrator as designed does not have the capacity to provide a desired target flow rate and dewpoint, the uncontrollable discharge is augmented with a purge gas, to increase the capacity of the dehydrator to achieve the target flow rate and dewpoint.